High-energy pulsed narrow-linewidth diffraction-limited rare-earth doped power amplifiers in the 950 to 1100 nm wavelength range and in the nanosecond regime generally require large mode area (LMA) fibers to mitigate Stimulated Brillouin scattering (SBS). However, typical LMA fibers with mode-field diameters larger than 20 μm are inherently multimode. To achieve a diffraction-limited output, several techniques are available such as low core numerical aperture, fiber coiling and selective doping.
High peak power amplification in rare-earth doped fibers suffers from nonlinear effects such as Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBS) [1, 2]. Core size and fiber length are the two parameters that are commonly varied to increase the threshold of these nonlinear effects. In the case of narrow linewidth and pulse width in the 10-ns range, SBS is the limiting factor for high peak powers. LMA fibers with core diameters of 10-15 μm yield nearly diffraction-limited output but their relatively small effective area (<200 μm2) allows only moderate high peak power levels. Core diameters greater than 20 μm are interesting but since the number of modes supported by the LMA fiber increases with the core diameter, the output of such a fiber becomes multimode. Lowering the numerical aperture of the core, defined as NA=√{square root over (ncore2−nfirst cladding2)}, n representing the refractive index, will reduce the number of modes, although a good control of the NA lower than 0.05 is a challenge for the MCVD (Modified Chemical Vapor Deposition) process.
Mode filtering by fiber bending is the most commonly used method to reduce the number of propagating modes in the fiber [3]. However, 100% higher-order mode suppression by this method is hard to obtain and the beam quality stays sensitive to variation in the mechanical and thermal stresses applied to the fiber.
A problem arises when relatively large core fibers are used. Indeed, the bending radii must be tightly controlled when the core size is relatively large (more than 30 microns for a typical core designed with a core numerical aperture in the range of about 0.05 to about 0.08) to minimize the bending losses of the first mode.
Another way to favor single-mode operation is to use selective doping [4-6]. In this case, the fundamental mode takes advantage of a higher gain compared to higher-order modes.
Short fiber lengths require a high concentration of rare-earth doping to achieve high gain amplification. This is often problematic since rare-earth doping increases the index of refraction. B2O3 or F can be incorporated to lower the refractive index to keep a low core NA.